Conversion of lower olefins to gasoline and/or distillate products is disclosed in U.S. Pat. Nos. 3,960,978 and 4,021,502 (Givens, Plank and Rosinski) wherein gaseous olefins in the range of ethylene to pentene, either alone or in admixture with paraffins, are converted into an olefinic gasoline blending stock by contacting the olefins with a catalyst bed comprising ZSM-M zeolite.
In a related manner, U.S. Pat. Nos. 4,150,062, 4,211,640 and 4,227,992 (Garwood, et al) disclose fixed-bed catalytic processes for converting olefins to gasoline and/or distillate components. In U.S. Pat. No. 4,456,779 (Owen, et al) and U.S. Pat. No. 4,443,185 (Tabak), incorporated herein by reference, operating conditions are disclosed for an olefin upgrading process for selective conversion of C.sub.3 + olefins to mainly aliphatic hydrocarbons.
Typically, the process recycles gas or liquid hydrocarbons from a high-temperature, high-pressure separator downstream of the catalyst bed back into the reaction zone where additional olefins are converted to gasoline and distillate products. If the reaction of the olefins in converting them to distillate and gasoline is allowed to progress adiabatically in the catalyst zone without any measures taken to prevent the accumulation of heat, the reaction becomes so exothermically accelerated as to result in high temperatures and the production of undesired products. The amount of recycle and the composition of the gas are critical to precise control of the reaction exotherm. Accordingly, in the conventional process, extra separation steps are included to separate a fraction from the reaction effluent which has the appropriate composition to function as a recycle liquid to the reaction zone. These additional separation steps represent a significant cost to the overall process.
In the process for catalytic conversion of olefins to heavier hydrocarbons by catalytic oligomerization using a medium pore shape selective acid crystalline zeolite, such as ZSM-5 catalyst, process conditions can be varied to favor the formation of either gasoline or distillate range products. At moderate temperature and relatively high pressure, the conversion conditions favor aliphatic distillate range product having a normal boiling point of at least 165.degree. C. (330.degree. F.). Lower olefinic feedstocks containing C.sub.2 -C.sub.8 alkenes may be converted; however, the distillate mode conditions do not convert a major fraction of ethylene. One source of olefinic feedstocks of interest for conversion to heavier fuel products is the intermediate olefin-rich naphtha or light oil obtained as a liquid product from Fischer-Tropsch conversion of synthesis gas.
A typical feedstock consists essentially of C.sub.3 -C.sub.6 mono-olefins with a minor amount of coproduced oxygenate from Fischer-Tropsch synthesis. These feedstocks are suitable for upgrading to more valuable heavier hydrocarbon; however, the organic oxygenated content may cause catalyst aging due to formation of coke during the conversion process. Typically, hydrogen is co-fed to the a fixed-bed catalytic reactor to reduce coking of the catalyst.
During the course of a single catalyst cycle, fixed-bed reactor inlet temperature must be raised to maintain the desired conversion of olefins to gasoline and/or distillate, and to maintain desired product liquid quality. Beyond a certain temperature, these objectives cannot be met and the catalyst must be regenerated. It is desirable to minimize the frequency of regeneration by decreasing the temperature aging rate. This reduces the inconvenience and cost of frequent regeneration, and may also extend the ultimate life of the catalyst, which experiences permanent activity loss over the course of many regenerations.
It is a main object of this invention to provide a continuous reactor system devised for upgrading olefins to a valuable heavy distillate fuel product.
It is a further object of this invention to provide an improvement to the downstream product recovery apparatus for the separation of a liquid recycle dilvent stream to the oligomerization reactor or conversion zone leading to a simplified and lower cost conversion process.